Depends. If your carrier implements RCS using simple HTTP auth, just download one of the 0 apps that bothered to actually implement RCS. Otherwise, your carrier may have an RCS app with an API. If your carrier doesn’t support RCS, you should probably treat Google’s RCS server the same way you treat WhatsApp, because it’s based on semi-open encryption with semi-open protocols on a closed-source, implementation-specific backend.
If you want Google to add Google Messages RCS to the SMS API, you’ll have to wait until they bother, which is probably “when someone forces them”. RCS is practically unused in the EU, the only place I’ve seen that cares about this kind of stuff, so you’ll have to contact your representatives.
That’ll only work on phones with Messages preinstalled, though, if you downloaded the app through the Play Store, it won’t be able to host any system APIs that truly expose RCS. It may also require a system update if they implement it by porting their RCS client to the Android operating system.